What is an anal fissure? Anal fissures are small tears in the thin tissue (mucosa) that lines the anus. (The anus is the opening at the end of your gastrointestinal tract). These tears can be painful and make you bleed. Sometimes, fissures rip slowly over time. Other times, they happen all of a sudden, from having to push a large stool or from childbirth. Anal fissures cause pain, bleeding.